Hermetic end seals for envelope of a metal vapor lamp

ABSTRACT

An alkali-metal-vapor discharge lamp has an inner vitreous material hermetic seal between an electrode stem and translucent tubular envelope and an outer metal seal between a metal end cap and the envelope. The cap is also sealed to the stem which extends from the tube.

United States Patent Delembre et al.

[ 1 Feb. 13, 1973 HERMETIC END SEALS FOR ENVELOPE OF A METAL VAPOR LAMP Inventors:

Assignee:

Filed:

Appl. No.:

Andre Emile Delembre, Asineres, France; Georges Roger Jarrige, deceased, late of Bagneux, France; by Jeanine Jarrige, administratrix, Germain Sucy-en-Brie; Dat Nhiep Nguyen, Chatenay-Malabrey, both of France ITT Industries, Inc., New York, N.Y. by said Delembre and Nguyen Aug. 13,1971

Foreign Application Priority Data Aug. 27, 1970- France .703l286 U.S. Cl.

Primary Examiner-Palmer C. Demeo Attorney-C. Cornell Remsen, Jr. et al.

[57] ABSTRACT An alkali-metal-vapor discharge lamp has an inner vitreous material hermetic seal between an electrode stem and translucent tubular envelope and an outer metal seal between a metal end cap and the envelope. The cap is also sealed to the stem which extends from the tube.

6 Claims, 2 Drawing Figures PATENTED FEB] 31975 Inventors ANDRE E. DELEMBRE GEORGES R. JARR/GE I By AT N. Nam/5 MA Horney HERM ETIC END SEALS FOR ENVELOPE OF A METAL VAPOR LAMP BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high-pressure alkalimetal vapor discharge lamps and particularly to highpressure sodium vapor lamps. More particularly, it relates to improvements in hermetic gas or vacuum tight seals which are made in manufacturing the lamps.

2. Description of the Prior Art The high-pressure alkali-metal-vapor discharge lamps comprise tubes of translucent polycrystalline alumina resistant to high pressure alkali-metal vapors. Such a tube is closed at its ends by pieces of refractory material also resistant to alkali-metal vapors. Pumping stems of refractory metal, which are used as electrode supports and current leads for the electrodes, pass through the refractory material pieces. Such a refractory metal is selected among the following metals and alloys thereof: niobium, zirconium, titanium, vanadium, tantalum, and preferably niobium and its well-known alloy with zirconium (1 percent).

Whatever refractory metal the end-seal is made of, the metal-alumina seal may be made either with eutectic mixtures of alumina and calcium oxide or a vitreous mixture of alumina, calcium oxide and magnesium oxide, or with active metals such as titanium, vanadium, zirconium or preferably combinations thereof.

If vitreous mixtures are used, their adherence to metal parts is weak, which may result in gas leakage from the discharge area to the outside during lamp life. In addition, due to high temperature and alkali-metal vapors, vitreous seal materials may become devitrified, which is another cause of leakage.

If active metal seals are used, they are made on the alumina tube end, which results in a short tight length compared with leakage line lengths, just a little longer than the tube thickness. In addition, at the start, with the pressure being low, a discharge impact spot may be located on a metal seal part, which can cause leaks,

SUMMARY OF THE INVENTION Therefore, a purpose of the present invention is to provide a tight closure of the discharge area which does not present the above mentioned drawbacks.

According to the invention, tight sealing of the discharge area is obtained by proper sealing of the tube and electrode stem with two end seals at each end of the tube. An outer end-seal is formed with a refractory metal cap sealed to the tube end by a combination of active metals and to the stem by brazing. An inner endseal is formed by a ring of alumina sealed to the tube and to the stem by a vitreous mixture.

Other purposes, features and advantages of the present invention will appear from the following description of a specific embodiment made in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic cross-section view of one of the two ends of the discharge area before melting, and

FIG. 2 shows the same elements after melting.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a schematic cross-section view of one end of the discharge tube before melting, the cross-section being obtained across a diametral plane. The gas or vacuum tight closure of the translucent polycrystalline alumina tube or envelope 1 is formed by two endseals. One end-seal 2 is made of a ring of highly pure alumina, which may be translucent or not. The other end-seal 7 is a cap made of refractory metal, preferably of niobium or niobium alloy. Those end-seals 2 and 7, through which the stem 4 passes, are sealed to the alumina tube 1 respectively by a ring 3 of vitreous material mainly constituted by alumina and calcium oxide, and by rings 10 of active metals such as titanium, vanadium and zirconium.

FIG. 1 also shows the electrode 5 secured to the stem 4 by a weld 9 made by any known process. The hole 6 is provided for evacuating or filling the discharge area 11. When the above mentioned elements have been located at each end of the tube 1, the assemblies are heated to a high temperature, under vacuum or inertgas-filled condition, to cause melting of the various rings. After having melted the rings, the seals are as shown in the FIG. 2 which includes the same elements as in the FIG. 1, the same reference numbers being used in both figures. It will be noted that the end-seal cap 7 may be sealed to the stem 4 by a seal 8 made according to any known process such as brazing, either before the ring melting operation, or at the same time. The area 12 between the two seals is preferably evacuated or filled with an inert gas.

FIG. 2 shows that, after melting, the ring 3 is shaped as shown by 3 and forms a tight hermetic seal between the end-seal 2 and the tube 1, on the one hand, and between the end-seal 2 and the stem 4, on the other hand. In a like manner, the rings 10 are then shaped as shown by 10 and form a tight seal between the tube 1 and the end-seal cap 7.

The advantages of the new seals are as follows:

1. Since the first seal is made with vitreous material, risks of hazardous arcing on the seal 10 (FIG. 2) are suppressed.

. If, after operation of a certain time, a devitrification phenomenon occurs for the seal 3 (FIG. 2), a tight seal is provided by the seal 10, (FIG. 2). However, the life-time of the seal 10 is longer than that which would be obtained if the end-seal 2 was not present. This is due to the fact that:

a. the seal temperature is lowered during operation which results in less severe thermal shocks during the life-time of the lamp;

. the seal is not in contact with alkali-metal vapor because the area 12 is evacuated or filled with inert gas, which reduces the risk of corrosion; and

c. there is no arcing on the seal.

The invention may also be utilized in any case where alumina-metal seals are in contact with alkali-metal vapors.

While the present invention has been described with respect to a specific embodiment, it is clear that the device is not limited to this example and that it can be modified or changed without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. A seal for a vapor discharge lamp comprising:

a tubular envelope of translucent vitreous material,

an electrode within one end of said envelope and having a stem extending out of said envelope,

a first seal of vitreous material between said stem and envelope,

a metal end cap at the outer end of said envelope,

said stem extending through said cap,

a second seal of refractory metal between said cap and said envelope, and

a further seal between said cap and stem.

2. The device of claim 1 wherein said first seal includes a ring of alumina and a vitreous mixture hermetically sealing said ring to said stem and envelope.

3. The device of claim 2 wherein said second seal includes a ring of refractory metals selected from the group consisting of titanium, vanadium and zirconium.

4. The device of claim 3 wherein said cap is of niobium and said further seal includes a brazing metal.

5. The device of claim 3 wherein said lamp includes an alk'ali-metal-vapor and the area between said first seal and metal cap is filled with an inert gas.

6. The device of claim 3 wherein said envelope is of poly-crystalline alumina and said vitreous sealing mixture includes a eutectic mixture of alumina and calcium oxide. 

1. A seal for a vapor discharge lamp comprising: a tubular envelope of translucent vitreous material, an electrode within one end of said envelope and having a stem extending out of said envelope, a first seal of vitreous material between said stem and envelope, a metal end cap at the outer end of said envelope, said stem extending through said cap, a second seal of refractory metal between said cap and said envelope, and a further seal between said cap and stem.
 2. The device of claim 1 wherein said first seal includes a ring of alumina and a vitreous mixture hermetically sealing said ring to said stem and envelope.
 3. The device of claim 2 wherein said second seal incluDes a ring of refractory metals selected from the group consisting of titanium, vanadium and zirconium.
 4. The device of claim 3 wherein said cap is of niobium and said further seal includes a brazing metal.
 5. The device of claim 3 wherein said lamp includes an alkali-metal-vapor and the area between said first seal and metal cap is filled with an inert gas. 